Polyimide resins have excellent thermal and chemical stabilities, low dielectric constants, and excellent planar contour-forming ability and, thus, have been attracting attention in the microelectronics industries. Polyimide resins have been widely used as materials for a surface-protective film or an interlayer insulating film for semiconductor devices, or as materials for multichip modules.
Generally, the process for forming a polyimide coating film in a desired pattern, for example, on a semiconductor device, is complicated. The process includes the steps of half-curing a polyimide precursor applied on, for example, a semiconductor device by heating, forming a pattern of a photoresist on the half-cured polyimide coating film, etching the polyimide coating film using the photoresist pattern as a mask, subsequently peeling-off the used photoresist pattern on the polyimide coating film, and subjecting the polyimide coating film on the semiconductor device again to heat treatment for imparting desired physical properties to the polyimide coating film. In such a complicated process, setting and controlling of various conditions for each step are difficult to perform satisfactorily, so that reproducibility is likely to be poor with respect to the physical properties of the resultant coating films. Further, the process has other drawbacks in that the pattern resolution is unsatisfactory because of indirect patterning, and that a hazardous substance, such as hydrazine, is used in the etching step.
Therefore, in recent years, for obtaining a pattern of a polyimide coating film, a method has been proposed, in which a polyimide precursor containing a photopolymerizable functional group is used. This method comprises forming a coating of a photosensitive composition containing the above polyimide precursor, a photopolymerization initiator, etc., photo-curing the formed coating, subjecting the photo-cured coating to developing to obtain a pattern coating, and then heating the pattern coating to convert the polyimide precursor to a polyimide resin. The polyimide precursor used in such a technique is generally referred to as "photosensitive polyimide precursor". The technique is described in detail in R. Rubner, H. Ahne, E. Kehn and G. Kolodziej, Photographic Science and Engineering, Vol. 23, No. 5, 303 (1979). By employing this technique, the drawbacks in the above-mentioned conventional process using a non-photosensitive polyimide precursor polymer have been overcome. Therefore, employment of this technique in obtaining patterns of polyimide coating films has been increasing.
Recently, however, demands for higher resolution in patterning a polyimide on a semiconductor device or the like have increased. In conventional processes using a non-photosensitive polyimide precursor polymer, the resolution in patterning is not so high and, therefore, semiconductor devices themselves and processes for manufacturing the same have been designed so as to take this into account. On the other hand, along with use of a photosensitive polyimide precursor polymer, which is capable of providing higher resolution in patterning, new semiconductor devices and processes, which utilize patterns of high resolution, have been developed.
The following is an example of utilization of high resolution polyimide patterns. In the manufacture of memory elements etc., in order to increase productivity, commonly employable circuits are first formed and, after inspection of the memory elements etc., unnecessary circuit portions are cut-off. In a conventional process using a non-photosensitive polyimide precursor polymer, this cutting step is conducted before forming the polyimide patterns. On the other hand, with the use of a photosensitive polyimide precursor polymer, it has become possible to obtain high resolution patterning of the polyimide. Therefore, in forming polyimide patterns on circuits, appropriate holes are first formed in the polyimide pattern film, so that cutting of unnecessary circuit portions can be performed through the holes after the formation of the polyimide pattern film. The cutting-off of unnecessary circuit portions after forming polyimide patterns enables the yield of products to be improved, since the timing of the cutting-off of the unnecessary circuit portions is closer to the finishing of fabrication for final products.
In such applications of polyimide precursors as described above, the holes to be utilized for cutting unnecessary circuit portions are required to be small for achieving high density circuit integration of the elements. Accordingly, the demand for photosensitive polyimide precursor compositions capable of providing polyimide patterns having higher resolutions than those currently achievable has increased. With a photosensitive polyimide precursor capable of providing a high resolution pattern, a wide process margin for patterning can be achieved, which is necessary for obtaining a high degree of integration and a high accuracy in the production of semiconductor devices. "Wide process margin for patterning" refers to a process that furnishes patterns that changes little with changes in process conditions, such as period in patterning, temperature, etc. Therefore, the higher the resolution, the better the results. This can also apply to the use of polyimide patterns to produce other elements, such as multichip modules etc. Accordingly, the demand for photosensitive polyimide precursor compositions, which are capable of providing polyimide patterns of high resolution and high accuracy, has increased. Since the required polyimide films tend to be thick and the required printed circuits tend to be more multiple in layer structure and more dense, the production of high resolution polyimide patterns is technically demanding.
Further, in recent years, better mechanical properties, especially a high elongation, have been desired with respect to polyimide coating films. In the conventional method for the production of, for example, semiconductor devices, a polyimide coating film is unlikely to suffer from stress, since there is conventionally employed a process in which a semiconductor chip having a polyimide film formed thereon is placed on a lead frame, followed by bonding therebetween, so that the polyimide film is unlikely to suffer from being loaded. However, when the recent mode of the method of the production of semiconductor devices is used, a polyimide coating film is likely to suffer from high stress since, in accordance with the requirement for miniaturization of semiconductor devices, a lead frame is placed on the semiconductor chip having the polyimide pattern formed thereon, in contrast to the case of the conventional mode, followed by bonding of the lead frame and the semiconductor chip, so that the polyimide film is inevitably likely to suffer from being loaded. Furthermore, regarding the production of multichip modules, electrical connector pins have been miniaturized and, along with the increased density of integrated circuits in the modules, portions of the polyimide coating film which serve to attach the pins thereto have become small in area, thereby causing the pin-attaching portions of the polyimide coating film to suffer from excessive stress. For coping with this problem, photosensitive polyimide precursor compositions capable of providing polyimide film patterns having a high elongation have been desired.
It has been known that when a polyimide resin is subjected to boiling testing, the properties of the polyimide resin are likely to deteriorate, so that the mechanical strengths thereof and the adhesion strength thereof to substrates are lowered, leading to a lowering in reliability of semiconductor devices produced using a polyimide resin. Accordingly, the demand for photosensitive polyimide precursor compositions capable of providing polyimide patterns having a high water resistance, has increased.
As described above, in order to cope with various problems occurring in connection with the development of integrated circuits of high density, high accuracy and high multiplicity in layer structure, the demand for photosensitive polyimide precursor compositions capable of providing polyimide pattern films having good mechanical properties including high elongation, high water resistance, and high pattern resolution, has increased.
Compounds referred to as "photosensitive polyimide precursor" can be grouped in a number of classes, according to their polymer backbone structures, types of photosensitive groups, etc. The compounds are described in detail in "Polymers for Microelectronics", Proceedings of the International Symposium on "Polymers for Microelectronics--Science and Technology--" (PME '89), Tokyo, Japan, Oct. 29 to Nov. 2, 1989, edited by Y. Tabata et al. [VCH: Weinheim, N.Y., Cambridge and Basel (1990)], pp. 789-810.
Examples of photosensitive polyimide precursors and systems using the same include a polyimide precursor having a polyamide chain with photosensitive groups bonded thereto by ionic bonds (Japanese Patent Publication No. 59-52822 by Hiramoto et al.); a mixture system in which a reactive monomer is mixed with a polyamide acid ["Kobunshi-Gakkai Yokoshu (Polymer Preprints, Japan), Vol. 39, No. 8, pp. 2397 (1990)" by Sashida et al.]; a system in which a polyamide acid partially esterified with a photosensitive group is adapted for development in an aqueous system (Japanese Patent Application Laid-Open Publication No. 3-220558, corresponding to EP 421,195 A, by A. E. Nader et al.); and the like. However, with a system using a polyimide precursor containing carboxylic acid residues, the photosensitive composition is not satisfactorily stable, leading to poorly reproducible results. That is, the conventional polyimide precursors are not useful because a difference in solubility between light-exposed portions and non-exposed portions of the polyimide precursor is not satisfactory, so that not only is the polyimide precursor of the light-exposed portions likely to be partially dissolved at the time of development, but also the precursor polymer thus dissolved precipitates on the developed pattern film upon contacting with a rinsing liquid. Therefore, high-resolution polyimide pattern films cannot be obtained with high accuracy and reproducibility.
In this situation, the present inventors have conducted extensive and intensive studies with respect to a polyimide precursor having an aromatic polyamide ester structure, which can obviate the above-described drawbacks of the prior art.
Photosensitive compositions using a polyimide precursor having an aromatic polyamide ester structure have been known and disclosed in, e.g., R. Rubner et al., "Photograph. Sci. Eng., No. 23, pp. 303 (1979)"; M. T. Pottinger et al., "The 38th Electric Components Conference, pp. 315 (1988)", etc. In each of these compositions, a polyamide ester having only 2-methacryloyloxyethyl group (containing a terminal ethylenic double bond) bonded thereto by ester linkage is employed.
Polyamide esters having groups (containing no terminal ethylenic double bond) bonded thereto by ester linkage have also been known, which are disclosed in, e.g., Okabe et al., "Kobunshi-Gakkai Yokoshu (Polymer Preprints, Japan), Vol. 41, No. 2, pp. 357 (1992)". A photosensitive composition using a polyamide ester having bonded thereto groups containing no terminal ethylenic double bond by ester linkage is disclosed in Matsuoka et al., Japanese Patent Application Laid-Open Publication No. 61-72022 (corresponding to Canadian Patent (A) No. 1 246 291).
The compositions disclosed by R. Rubner et al. and Pottinger et al. are similar to the above-mentioned photosensitive polyimide precursor compositions which are currently used mainly in the fields of semiconductor devices etc., but are not satisfactory with respect to not only resolution, but also elongation and water resistance of polyimide pattern coating films obtained therefrom, as described above. The above-mentioned composition disclosed by Matsuoka et al. has drawbacks in that the sensitivity to U.V. light is low, due to the low degree of photo-crosslinking even upon light exposure, and that the difference in solubility between light-exposed portions and non-exposed portions is small, so that not only are the light-exposed portions of the photosensitive composition likely to be partially dissolved at the time of development, but also the polyimide precursor thus dissolved precipitates on the developed pattern film upon contacting with a rinsing liquid, leading to a lowering in resolution of the resultant pattern. Thus, the known compositions are inappropriate for practical use.